In order to provide communications service to a customer, it is commonplace to extend a small pair size metallic conductor cable or service wire, as it is called, from a distribution cable to the customer's premises. The demand for such service wires seemingly has not abated notwithstanding increasing amounts of activity to provide optical fiber to the home.
Such a service medium must meet several requirements. Inasmuch as it has one end thereof adjacent to the customer's premises, it must be flame retardant. Further, it must have suitable mechanical strength to withstand the rigors of installation when it is plowed into the ground and to be able to resist compressive forces imparted to the cable during the installation stage and during its use. Typically, a plow used for installation has a vibratory cutting block which is referred to as a plow share. As the plow share is vibrated, the cable or wire is moved through a tube attached to the blade and into a trench along a curved path having a sharp radius such as several inches, for example.
Also, because the service wire is buried, it is required to include waterblocking provisions. These are required to prevent the longitudinal flow of water along the service wire to connection points. Typically, this has been accomplished in these kinds of wires by using a waterblocking, grease-like, filling composition of matter. This last-mentioned material is intended to fill interstices among conductors of a core.
A typical prior art service wire used in buried applications includes copper conductors having a filling material disposed thereabout and being enclosed by a dielectric core wrap material. A filling material is provided over the core wrap and is enclosed by a metallic shield having a longitudinal overlapped seam and a plastic jacket. Typically, the metallic shield is bronze. One commonly used service wire has an outer diameter of about 0.69 cm.
As far as strength properties are concerned, it has been commonplace to provide such a service wire with a metallic shield. The shield which also may be made of steel, for example, provides the requisite tensile strength and also functions to dissipate electromagnetic energy. Disadvantageously, shields have the effect of reducing manufacturing line speeds. Further, a shield must be electrically bonded or grounded with any one of a number of commercially available grounding clamps at the customer end and at the distribution cable end. Bonding of connection devices to the shield is time consuming. Because of the presence of the shield, there is a greater likelihood of a lightning strike with it than without it. Still further, a shield increases considerably the capacitance of the service medium. Also, as a shielded wire is handled and plowed into the ground, overlapping longitudinal edge portions of the shield may project into the plastic jacket which may cause the jacket to rupture.
Communications cables or wires which are strung between poles or those which are buried in the ground may be subjected to abuse such as, for example, attack by animals, mechanical abrasion and crushing. Attacks by gophers on buried cable and wire and by squirrels on aerial cable have been a continuing concern in some areas of the country. An excellent discussion of the problem of animal attack was presented at the 25th International Wire and Cable Symposium. A written version of that presentation appears beginning at page 117 in the proceedings of that conference, being authored by N. J. Cogelia, G. R. LaVoie, and J. G. Glahn and being entitled "Rodent Biting Pressure and Chewing Action and Their Effects on Wire and Cable Sheath". It has been found that an effective way to protect directly exposed cables from rodent attack is to wrap them in a metallic shield. See also U.S. Pat. No. 4,874,219 which issued on Oct. 17, 1989 in the names of C. J. Arroyo and P. D. Patel. However, the inclusion of a metallic shield restricts the manufacturing line speed, as pointed out hereinbefore and, of course, adds to the cost of the product.
Certainly, any new service wire must be designed with rodent protection being considered. However, what also is known is that these kinds of service wires are attacked by rodents only in well defined geographical areas of the United States and foreign lands. As a result, wires which may require a shield to provide protection against rodents may not need to be used as a universal service distribution wire in other areas of the United States and abroad.
One commercially available service wire is manufactured without a shield but with a circular configuration with strength members numbering on the order of about eight being embedded in the jacket. A grease-like filling material fills the core of this cable. Craftspersons in the field are not overly fond of the use of such materials because of housekeeping problems encountered during installation such as during splicing.
What is needed and what seemingly is not provided in the prior art is a service wire which is suitable for buried use in providing service to a customer. The sought-after service wire should have suitable flame retardancy and waterblocking provisions other than grease-like filling materials. Further, it must be relatively economical to manufacture.